In general, an ethylene-vinyl alcohol copolymer (hereinafter, referred to as EVOH) is excellent in transparency, gas barrier properties, aroma retention, solvent resistance, oil resistance and the like and has been used for various packaging materials such as a food packaging material, a pharmaceutical packaging material, an industrial chemical packaging material and a agricultural chemical pack aging material making the most use of such properties. The EVOH often subjected to thermal stretching treatment for the purposes of deformation to a container for practical use and the improvement of mechanical strength and the like. Further, in recent years, when molding a container from a multi-layer sheet containing EVOH, a container with a deep bottom has been also prepared from the viewpoints of the variety of container shape and design properties and EVOH having excellent moldability, so that the appearance, barrier properties and strength of the container after molding are favorable, is desired for molding such deep containers.
However, EVOH is inferior to polypropylene and polystyrene in thermal stretching properties and as measures therefor, (1) the method of adding a plasticizer to EVOH (for example, see JP-A-53-88067 and JP-A-59-20345) and (2) the method of mixing polyamide resin EVOH (for example, see JP-A-52-141785 and JP-A-58-36412) have been suggested. Also, on the other hand, there are also suggested (3) the method of using a resin composition wherein EVOH having a low glass transition temperature, which has relatively favorable stretching properties, is used together (for example, see JP-A-61-4752, JP-A-60-173038, JP-A-63-196645, JP-A-63-230757, JP-A-63-264656 and JP-A-2-261847) and (4) the method of mixing 3 types of EVOH (for example, see JP-A-2001-31821 and JP-A-2001-31823). Furthermore, studies have been conducted regarding (5) adding an ethylene-(meth)acrylic acid copolymer to EVOH to improve the thermal stretching properties thereof at low temperatures (for example, see JP-A-11-99594) and (6) grafting an epoxy compound to EVOH by a melting reaction to improve thermal moldability for forming a container and stretching properties (for example, see WO 02/092643 and JP-A-2003-327619).
However, when the present inventors studied the above methods in detail, it was found that in method (1), the gas barrier properties decreased and in method (2), long run melt moldability decreased. In method (3), the improvement of thermal stretching moldability is confirmed to a certain degree, but because EVOH of different composition and structure are mixed, the compatibility is not completely homogeneous. Also, EVOH tends to be influenced by fluctuation of extrusion conditions and thermal stretching molding conditions and defective articles are inevitably produced in the case that films, cups, trays and bottles are continuously molded by stretching. In method (4), continuous moldability is improved but defective articles are inevitably produced in the case that a container having a large drawing ratio, which requires high stretching properties, is formed. In method (5), moldability in relatively low temperatures is improved but long run melt moldability may decrease. Also, it was found that the appearance, barrier properties and strength of a deep container having a drawing ratio of at least 2.5 were not at all studied. In method (6), because EVOH and an epoxy compound are reacted in a melted condition, miscellaneous side reaction products are inevitably produced and decrease in long run melt moldability and problems of safety and sanitation may occur. Furthermore, it was found that the appearance, barrier properties and strength of a deep container having a drawing ratio of at least 2.5 were not at all studied. In this way, EVOH having excellent moldability, so that the appearance, barrier properties and strength of the container after molding are favorable even when a deep container is formed, is desired.
Also, besides those described above, EVOH has the following problems depending on the use.
For example, EVOH is used for multi-layer containers prepared by laminating polyester resin (mainly polyethylene terephthalate, hereinafter referred to as PET) on both faces of an EVOH layer in order to improve properties such as humidity resistance of the container, barrier properties of carbon dioxide and aromatic components and mechanical properties. Recently, such multi-layer containers are attracting attention as pressure resistant bottles for carbonated soft drinks and alcoholic drinks.
PET has excellent transparency and stiffness, moderate gas barrier properties and aroma retaining properties and is widely used in containers for carbonated drinks and soft drinks. However, the gas barrier properties thereof are insufficient for uses that require high gas barrier properties, such as containers for beer and wine, and as described above, PET can be used as an excellent gas barrier container by laminating with an EVOH layer. However, usually, a thermoplastic polyester resin such as PET and EVOH are poor in adhesion and in order to increase interlayer peeling strength and interlayer peeling resistance, a specific adhesive resin must be between the layers.
However, recently, PET is recycled and reused and in the case that an adhesive resin is present between the layers, separation of PET and EVOH becomes difficult. As a result, there is the problem that the recycled PET deteriorates in quality and therefore has difficulty being accepted in the market.
A multi-layer container wherein polyester resin (PET) is laminated on both sides of the EVOH layer without using an adhesive resin is suggested (for example, see JP-A-61-173924). However, because an adhesive resin is not used, interlayer separation may occur between the EVOH layer and the PET layer while using as a container. As measures therefor, there are suggested (7) the method of mixing several kinds of EVOH (for example, see JP-A-11-348196, JP-A-2001-236919, JP-A-2002-210888 and JP-A-2002-210889), (8) the method of using EVOH of a low hydrolysis degree (for example, see JP-A-11-348197) and (9) the method of mixing other resin (for example, see JP-A-11-79156, JP-A-2002-210887 and JP-A-2002-210890). Also, (10) the method of grafting an epoxy compound to EVOH by a melting reaction to improve thermal moldability for forming a container and stretching properties (for example, see JP-A-2003-320600) is suggested.
However, although interlayer impact peeling resistance is improved by each of the above methods, in method (7), the different kinds of EVOH are not completely compatible and transparency decreases and pressure resistant strength tends to decrease, as stretching is uneven. In method (8), decrease of barrier properties may occur and in method (9), decrease of transparency may occur. In method (10), transparency of the body is improved but because an EVOH composition obtained by reacting EVOH and an epoxy compound in a melted state is used as the middle layer, miscellaneous side reaction products are inevitably produced and decrease in long run melt moldability and problems of safety and sanitation may occur. Also, the transparency of the bottom and neck of a bottle wherein the layer of EVOH becomes thick is not at all considered. Furthermore, in recent years, there is a tendency to reduce the amount of resin used in bottles from the viewpoint of resource saving and a bottle having favorable pressure resistance and small difference in pressure resistance strength, as all bottles cannot be tested for pressure resistance, is desired. Studies up to present have not considered pressure resistance or difference in pressure resistance strength. Desired is a bottle having favorable barrier properties, interlayer impact peeling resistance and transparency of the bottom and neck, high pressure resistance and small difference in pressure resistance strength.
EVOH is used for various packaging materials by laminating film of low-density polyethylene, polypropylene, nylon or polyester on both sides of EVOH, to maintain properties of EVOH such as gas barrier properties, aroma retaining properties and anti-discoloring properties of foods and compensating shortcomings of EVOH such as falling strength, thermal moldability and moisture resistance. Moreover, recently, in addition to packaging for food as described above, EVOH is used for containers such as bottles, tanks and drums for transporting and storing fuel having hydrocarbon as the main component.
However, in such uses, further improvement of fuel barrier properties is desired. For example, it is suggested that (11) a fuel container having EVOH as the middle layer, in which the outer layer thickness is larger than the inner layer thickness (for example, see JP-A-9-29904), and (12) a fuel container having as the middle layer EVOH containing a small amount of ethylene and having a specific metal salt (for example, see JP-A-2001-341535).
In recent years, due to tightening regulation regarding environmental pollution, high fuel barrier properties are necessary under the conditions of long term use and stability of the quality of the canister is strongly desired. However, in the methods of (11) and (12), fuel barrier properties may decrease after subjecting to heat shock and also, before heat shock, fuel barrier properties differ in each fuel container.
Utilizing its properties, EVOH is molded into films, sheets, tubes, cups, trays and bottles for packaging materials for food, pharmaceutical products, industrial chemicals and agricultural chemicals. Particularly, because most fatty foods such as meat and processed foods thereof are irregular in shape and size, EVOH is often used as shrink packaging in order to improve fresh storage and appearance of the contents. Therefore, a multi-layer shrink film which is excellent in thermal shrinking properties and gas barrier properties is desired. In order to improve such properties, it is suggested that (13) the method of mixing two kinds of EVOH having a different composition (for example, see JP-A-5-200865 and JP-A-2000-211068), (14) the method of mixing another resin in EVOH (for example, see JP-A-5-77352, JP-A-5-228996, JP-A-7-1685, JP-A-8-81610, JP-A-8-81570 and JP-A-2000-246843) and (15) the method of mixing a plasticizer with EVOH (for example, see JP-A-5-261815 and JP-A-5-200865).
However, in a multi-layer shrink film obtained by method (13), thermal shrinking properties and gas barrier properties are excellent, but because two kinds of EVOH having a different composition are mixed, compatibility is insufficient and areas of decreased transparency develop in some areas after thermal shrinkage. Also, because adhesion between the adhesive resin layer which adheres the EVOH layer and the thermoplastic resin layer and the EVOH layer decreases, there is the problem that interlayer separation (delamination) occurs in the multi-layer film after shrinking. In a multi-layer shrink film obtained by method (14), because a different resin is mixed, adhesion decreases and there is the problem that interlayer separation (delamination) occurs in the multi-layer film after shrinking. In a multi-layer shrink film obtained by method (15), the problems of decrease in barrier properties and delamination occur. Thus, a multi-layer shrink film that is excellent in stretching properties, thermal shrinking properties, gas barrier properties, transparency after thermal shrinkage and delamination resistance is desired.
The present invention aims to provide a new ethylene-vinyl alcohol copolymer, which has improved stretching properties and provides a molded article having excellent gas barrier properties, appearance and strength, and a molded article thereof.